1. Field of the Invention
The present invention relates generally to optical pickups used for recording and/or reproducing information signals and, in particular, to a biaxial actuator for an objective lens of an optical pickup used for recording and reproducing signals of a CD, MD, digital video disc, or other information recording medium.
2. Description of the Related Art
Conventionally, reproduction or recording of information signals with respect to a disc-form recording medium, such as an optical disc, for example a so-called compact disc (CD) or an optomagnetic disc, has been carried out using an optical pickup. This optical pickup includes a semiconductor laser as a light source, an objective lens, an optical system and a light detector.
In an optical pickup, a light beam emitted from the semiconductor laser is converged through the optical system onto the recording surface of an optical disc by the objective lens. A return light beam from the optical disc is split from the light beam emitted from the semiconductor laser and guided to the light detector by the optical system. The position of the objective lens in the optical axis direction is adjusted so that the light beam emitted from the semiconductor laser follows displacements of the optical disc in a direction intersecting orthogonally with the plane direction of the optical disc occurring as a result of warp of the optical disc and the like and is focused at the recording surface of the optical disc. At the same time, the position of the objective lens in a direction intersecting orthogonally with the optical axis is adjusted so that the position of a spot on the optical disc of the light beam emitted from the semiconductor laser follows any eccentricity of the optical disc and snaking of a track formed on the optical disc.
This adjustment of the focusing position and the spot position on the recording surface of the light beam emitted from the semiconductor laser is carried out by adjusting the position in the optical axis direction of the objective lens and the position in a direction intersecting orthogonally with the optical axis of the objective lens. An electromagnetically driven actuator is used for positional adjustment of the objective lens.
This actuator is called an objective lens actuator or a biaxial actuator and includes a bobbin on which the objective lens is mounted and on which a plurality of coils are wound, a plurality of elastic supporting members and a driving part that generates driving forces by passing currents through the coils of the bobbin.
The bobbin is so supported with respect to a fixed part by the plurality of elastic supporting members that the position of the objective lens in the optical axis direction, namely the focusing position, and the position of the objective lens in a direction intersecting orthogonally with the optical axis, namely the tracking position, are adjustable. An example of this biaxial actuator is described below with reference to FIGS. 8 and 9.
This kind of biaxial actuator is constructed for example as shown in FIG. 8. That is, a biaxial actuator 1, as shown in FIG. 8, comprises a lens holder 2 on the front end of which is mounted an objective lens 2a and a coil bobbin (not shown in the drawing) attached to this lens holder 2 by means of an adhesive or the like.
The above-mentioned lens holder 2 is supported with respect to a fixed part 3 movably in two perpendicular directions, namely a tracking direction perpendicular to the paper surface and a focusing direction shown with the mark Fcs, by two pairs of wires 4 having one end fixed to both sides of this lens holder 2 and the other end fixed to the fixed part 3.
Also, on the above-mentioned coil bobbin are wound a coil for focusing and a coil for tracking (not shown in the drawing). By passing currents through the coils, magnetic flux produced in the coils mutually acts on magnetic flux from a yoke attached to the fixed part 3 and a magnet attached thereto (not shown in the drawing).
Also, the rear ends of the above-mentioned wires 4 pass through this fixed part 3 and are soldered to a base plate 5. Here, as shown in FIG. 9, to suppress vibration of these wires 4, these wires 4 are fitted through dampers 6 passed through holes 3a, 3b in the fixed part 3. In the case of FIG. 9, the dampers 6 are in contact with the base plate 5.
With a biaxial actuator 1 constructed like this, and driving voltages being supplied to the coils from outside, magnetic flux produced in the coils mutually acts on magnetic flux from the yoke and the magnet and this coil bobbin is moved with respect to the tracking direction and the focusing direction Fcs. In this way, the objective lens 2a mounted on the lens holder 2 is suitably moved with respect to the focusing direction and the tracking direction.
When the lens holder 2 is moved with respect to the focusing direction and the tracking direction in this way, this lens holder 2 tries to vibrate in relation to the movement direction, but by the damping action of the dampers 6 provided near the rear ends of the wires 4 the vibration is suppressed. As a result, the lens holder 2 is stopped in a predetermined position in a stable state.
A biaxial actuator constructed as shown in FIG. 10 is also known. That is, a biaxial actuator 7 comprises a lens holder 2 on the front end of which is mounted an objective lens 2a and a coil bobbin (not shown in the drawing) attached to this lens holder 2 by means of and adhesive or the like.
The above-mentioned lens holder 2 is movably supported with respect to a fixed part 3 in two perpendicular directions, namely a tracking direction perpendicular to the paper surface and a focusing direction, at least one pair of sheet springs 8 having one end fixed to both sides of this lens holder 2 and the other end fixed to the fixed part 3.
Also, on the above-mentioned coil bobbin are wound a coil for focusing and a coil for tracking (not shown in the drawing). By passing currents through the coils, magnetic flux produced in the coils mutually acts on magnetic flux from a yoke attached to the fixed part 3 and a magnet attached thereto (not shown in the drawing).
In this case, the rear end vicinity of each of the above-mentioned sheet springs 8, as shown in FIG. 11, has a crank part 8a bent into a crank shape toward the inner side and an extension part 8b extending from further forward than this crank part 8a toward the outer side and extending to the rear, and is provided with a projecting part 8c projecting from the rear end of this sheet spring 8 between the above-mentioned crank part 8c and the extension part 8b. A slit 8e is set in a direction intersecting orthogonally with the optical axis direction of the objective lens 2a between this projecting part 8c and the crank part 8a. 
An anti-vibration tape 9 is affixed as a damper so as to completely cover this crank part 8a and the extension part 8b, and the projecting part 8c and slit 8e extending therebetween.
When the lens holder 2 is moved with respect to the focusing direction and the tracking direction in this way, this lens holder 2 tries to vibrate in relation to the movement direction, but by the damping action of the anti-vibration tape 9 provided in the vicinity of the rear ends of the sheet springs 8 the vibration is suppressed. As a result, the lens holder 2 is stopped in a predetermined position in a stable state.
However, in a biaxial actuator 7 of this kind of construction, when moving the lens holder 2 in the focusing direction, there have been the following kinds of problem. That is, FIGS. 12(a) and 12(b) show a state of the lens holder 2 being moved in the focusing direction and brought close to a disc D, and FIGS. 13(a) and 13(b) show a state of the lens holder 2 being moved in the focusing direction and moved away from the disc D.
As shown in FIGS. 12(a) and 12(b), when the lens holder 2 is brought close to the disc D, that is, moved in the direction shown by the arrow H upward in the drawing, a force F1 arises in upper and lower springs 8-1 and 8-2 because the place of the slit 8e (FIG. 11) contracts and expands in relation to the X direction (FIG. 12(a)), and in connection with the distance L a moment M1 (M1=F1L) acts on the lens holder 2. As a result of this, as shown in FIG. 12(b), the optical axis of the objective lens 2ashifts and so-called plus side tangential skew occurs.
Also, when the lens holder 2 is moved for focusing in the direction of the arrow I, as shown in FIG. 13, so as to be moved away from the disc D, oppositely from that discussed above, in the upper and lower springs 8-1 and 8-2 the place of the slit 8e (FIG. 11) contracts and expands in relation to the X direction (FIG. 13), a force F2 arises, and in connection with the distance L a moment M2 (M2=F2L) acts on the lens holder 2.
In this case also, as shown in FIG. 13(b), the optical axis of the objective lens 2a shifts and so-called minus side tangential skew occurs.
Because of this, in the biaxial actuator 7, there has been the problem that the occurrence of so-called dynamic skew, which is this kind of inclination of the optical axis of the lens holder 2, causes the optical performance of the optical pickup, such as the signal reading performance, to deteriorate.
In particular, of this dynamic skew, because the level of tolerance of the tangential skew described above in carrying out accurate signal reading and so on is small, prevention of the occurrence thereof has been sought.
An object of the present invention is to solve the problems associated with the aforementioned conventional optical pickup assembly.
More particularly, the present invention has an object of providing a biaxial actuator wherein when the objective lens is moved in the focusing direction, shifting of the optical axis of the objective lens is prevented, thereby reducing the occurrence of tangential skew and improving the optical performance of the actuator.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The above-mentioned objects, according to this invention, maybe achieved by an actuator for an objective lens comprising a lens holder for holding the objective lens, a coil bobbin wound with a focusing coil and at least one tracking coil, the coil bobbin being fixed on the lens holder, magnet means for generating a magnetic field to the focusing coil and the tracking coil for driving the lens holder with the coils, and a plurality of elastic supporting members for movably supporting the lens holder in a direction parallel to the optical axis of the lens and a direction perpendicular to the optical axis of the lens, first end portions of the supporting members being fixed to the lens holder and second end portions of the supporting members being fixed to a fixed member. The supporting members are arranged such that, in a direction parallel to the optical axis of the lens, a first distance between the first end portions of the supporting members is shorter than a second distance between the second end portions of the supporting members.
According to the construction described above, the mutual spacing of the pair of elastic supporting members lined up in the optical axis direction of the objective lens is narrow at the lens holder side and wide at the fixed part side. By this construction, the biaxial actuator is given behavioral characteristics such that when the lens holder is moved in the optical axis direction, the so-called focusing direction, minus side tangential skew occurs when the lens holder is brought close to the disc-form recording medium, and plus side tangential skew occurs when the lens holder is moved away from the disc-form recording medium. Because these kind of behavioral characteristics are opposite to the behavioral characteristics occurring when expanding/contracting portions are provided in the elastic supporting members, reciprocal characteristics cancel each other out, and shifting of the optical axis of the objective lens is not caused during focusing movement.
In a preferred embodiment, the supporting members of the actuator each have a portion that expands and contracts in a direction parallel to a longitudinal direction of the supporting member. The expanding and contracting portions are provided on the second end portions of the supporting members. The expanding and contracting portions have a first portion that does not displace during focusing movement of the lens holder, a second portion disposed more on the lens holder side than the first portion, and a third portion connected to the first portion and second portion that displaces together with the second portion during focusing movement of the lens holder. The expanding and contracting portions each comprise a slit that is provided between the first portion and the second or third portion, and a viscous element is applied to the expanding and contracting portions. The supporting members are made of an electrically conductive material and are each electrically connected to at least one of the focusing coil and the tracking coil.
In another aspect of the present invention, the objects of the invention maybe achieved with an actuator for an objective lens, comprising a lens holder for holding the objective lens, a coil bobbin wound with a focusing coil and at least one tracking coil, the coil bobbin being fixed on the lens holder, magnet means for generating a magnetic field to the focusing coil and the tracking coil and for driving the lens holder with the coils, and supporting means for movably supporting the lens holder in a direction parallel to the optical axis of the lens and a direction perpendicular to the optical axis of the lens, the supporting means comprising two pair of elastic supporting members arranged at both sides of the objective lens and arranged so that the elastic supporting members in each pair are aligned in a direction parallel to the optical axis of the lens, first end portions of each of the supporting members being fixed to the lens holder and second end portions of each of the supporting members being fixed to a fixed member. A first distance between the first end portions of the supporting members in each pair of supporting members is shorter than a second distance between the second end portions of the supporting members in each pair, and the supporting members each have a portion that expands and contracts in a direction parallel to a longitudinal direction of the respective supporting member.